1. Field of the Invention
The invention relates to the use of an additive containing zinc oxide for improving the solvent resistance or other properties, such as resistance or drying behavior, of water-based binder systems, to the additive, and to an aqueous coating material comprising the additive.
2. Discussion of Background Information
Zinc oxide is already used for various purposes in water-based coating materials. Its use as a white pigment is well known. In wood coatings, ZnO is used, besides other zinc salts, as an additive for preventing the bleeding of tannic acid. Zinc oxide is also used in primers on metals, as an adhesion promoter. For these applications, zinc oxide particles with a diameter of more than 1 μm are employed.
Zinc oxide and other metal oxides are also used in coating materials in order to improve UV protection, solvent resistance, and surface hardness. WO 2008/049679 describes the use of nanoparticles for coating materials on an organic solvent basis, in order to achieve an improvement in scratch resistance and in flexibility. U.S. Pat. No. 6,342,556 describes the use of ZnO particles having a diameter of 10 to 80 nm in water in combination with coating materials for improved UV protection of wood.
WO 2005/071002 relates to the use of a dispersion comprising ZnO having a BET surface area of 10 to 200 m2/g as transparent UV protection in coating materials. The ZnO particles described in that application are pyrogenically prepared particles which in dispersion have an average secondary particle size of less than 300 nm. Although these pyrogenically produced particles do possess a specific surface area in the order of magnitude of nanoparticulate zinc oxide, they cannot be used to produce transparent coats. The additive specified in WO 2005/071002 is therefore unsuitable for use in transparent coating systems.
WO 2005/071029 uses zinc oxide having a particle diameter of less than 100 nm in order to improve the surface hardness and the scratch resistance of polyurethane (PUR) coating materials. WO 2006/023064 relates to the use of ZnO dispersions in PUR binders for producing scratch-resistant floor coatings. WO 2006/023064 describes the production of scratch-resistant wood-preservative coating materials through the use of ZnO dispersions in acrylic binders. A mixture of Al2O3 microparticles and nanoparticles is used in PU resins and melamine/formaldehyde resins for improving the surface hardness (WO 03/040223).
WO 2006/023064 describes the use of 2 to 20% nanoscale zinc oxide in acrylate-containing coating materials and coatings, in order to improve the adhesion, tannin resistance or corrosion resistance, for example. Since the fraction of the relatively expensive ZnO is more than 2%, the composition is economically irrelevant for the top coat sector. WO 2006/023065 relates to coating compositions which comprise microscale TiO2 pigments and nanoscale metal oxide such as ZnO, the intention being that the nanoscale metal oxide should ensure a uniform spacing of the TiO2 pigments in the coat.
WO 2005/119359 and US-A1-2003/0180466 describe film-forming compositions which comprise binder, nanoparticle, surfactant, and a polymeric dispersant, in order to improve the abrasion resistance of substrates coated with such compositions.
One of the means employed for increasing the solvent resistance of coatings is their subsequent crosslinking with UV initiators, as shown in U.S. Pat. No. 4,121,985, for example. In accordance with JP-A-62-28496, solvent-resistant coats are produced by the use of acidically/basically crosslinking binder constituents. In addition to the use of UV initiators, functional silanes as well are employed for the postcrosslinking of polymer coats, as described in DE-T2-69426674, for example. JP-A-11012426 describes resin compositions comprising carboxylated acrylic resin and inorganic particles with oxazoline groups on the surface, in order to obtain coatings having good solvent resistance.
WO 2008/064114 relates to the use of ZnO particles having a diameter of less than 1 μm in an acrylate hybrid coating material for the purpose of improving the solvent resistance, the moisture resistance, and the UV resistance. JP-A-57123271 relates to clearcoat compositions, such as acrylic varnishes, to which SnO2 powder or In2O3 powder with a diameter<100 nm is added, for purposes including that of improving the solvent resistance. On account of the costs of the powders employed, especially of the In2O3 powder, this approach does not have much economic interest.
A UV post-crosslinking for improving the solvent resistance necessitates an additional operating step. Moreover, chemical postcrosslinking results in the embrittlement of the coating film. This places limits on its use, particularly for applications where high elasticity is needed.
There is therefore a demand for additives for binder systems, more particularly coating systems, with which the solvent resistance of water-containing binder systems can be significantly improved without detracting from the overall profile of the binder system through the addition of large quantities. In this context, even quantities of less than 2% by weight ought, when added, to achieve a significant effect, in order to allow a reduction in production costs and to have as little detrimental effect as possible on the other properties of the binder system, particularly of a coating material.
Surprisingly it has been found that the combination of at least two inorganic nanoparticle dispersions results in a synergistic boost to the solvent resistance of the dried or cured binder system. The particle mixtures are mixtures of zinc oxide with at least one other metal oxide, semimetal oxide or oxo salt.